1. Field of the Invention
The present invention relates to a method for preparing sulfonated polystyrene for a polymer electrolyte for a fuel cell, and more particularly, to a method for preparing sulfonated polystyrene having good proton conductivity, a sulfonated polystyrene polymer electrolyte prepared according to the method, and a fuel cell comprising the sulfonated polystyrene polymer electrolyte.
2. Description of the Related Art
Fuel cells as electrochemical cells convert energy that is generated by an oxidation reaction of fuel to electrical energy. On an anode of a fuel cell, organic fuel such as methanol, formaldehyde, or formic acid is oxidized to carbon dioxide, and on a cathode, air or oxygen is reduced to water. A high specific energy of organic fuel renders the fuel cell more attractive. For example, the specific energy of methanol is 6232 wh/kg.
Fuel cells comprise a membrane electrode assembly (MEA) including an anode layer, a cathode layer, and a polymer electrolyte membrane (PEM) that is interposed between the two electrode layers and plays a role as a proton transfer medium. As the conductive polymer electrolyte of a fuel cell, a fluorine-containing polymer electrolyte membrane such as a perfluoro carbon sulfonic acid membrane (Nafion™ manufactured by DuPont Company) has chemical stability, high ionic conductivity and good mechanical properties, and is generally used.
However, a fluorine-containing polymer electrolyte has a disadvantage in that it is prepared by a complicated process, and has a high manufacturing cost. Further, since a fluorine-containing polymer electrolyte has a heat-resistance limit of less than 100° C., a cooling system for reforming gas and a removing system for carbon monoxide are needed when it is used as a power source for an automobile, a commercial small-sized power plant, or a portable power plant. The proton conductivity deteriorates and infiltration of methanol occurs at a high temperature of more than 80° C. or under low humidity of less than 60%.
Therefore, a sulfonated polymer such as sulfonated polyimide, polystyrene, polyphenylene, or polyetheretherketone (PEEK) has been researched as a polymer electrolyte membrane to replace fluorine-containing polymer electrolyte membranes. The sulfonated polystyrene is prepared through post-sulfonation of polystyrene. For example, Dais Analytical Corp. developed a sulfonated styrene-ethylene/butylene-styrene triblock polymer as a polymer electrolyte. However, the post-sulfonation has a disadvantage in that it is difficult to control the degree of sulfonation and it is difficult to obtain a material having various structures.
Japanese Patent Laid-open No. 97-102322 discloses a sulfonated polystyrene-graft-ethylene tetrafluoroethylene (ETFE) polymer comprising a copolymer of a fluorocarbon-based vinyl monomer and a hydrocarbon-based vinyl monomer as a main chain, and a hydrocarbon chain having a sulfone group as a side chain. The ETFE polymer can be prepared at a low cost, has enough mechanical properties for a polymer electrolyte, and can be improved in terms of conductivity by introducing many sulfone groups. The main chain of the copolymer of a fluorocarbon-based vinyl monomer and a hydrocarbon-based vinyl monomer has high oxidation resistance properties, but the hydrocarbon side chain is easily oxidized and deteriorated. Therefore, the ETFE polymer has insufficient oxidation resistance and durability for a polymer electrolyte for a fuel cell.